Detailed characterizations of human tumor cell lines are being conducted to determine the suitability of individual cell lines for inclusion in the in vitro anticancer drug screen, as well as to aid in the evaluation of data from the screen. Characterizations include histological and ultrastructural observations and immunocytochemical testing with panels of antibodies directed against tissue and tumor specific antigens, and antigens associated with specific biological properties (multiple drug resistance, oncogene encoded products, etc.). The influences of the growth environment (in vitro monolayer or three dimensional, in vivo xenograft) and other cultivation parameters on expression of cell line characteristics are also being studied.